Determining the shape of an intrusion: The Tallarook granite

Abstract

Field-based studies can place constraints on the shape of granite intrusions with the results being complementary to the less direct approach using geophysics. The Tallarook plateau in central Victoria provides the necessary outcrop and topographic relief to allow mapping of the outer boundary of the granite in detail, and to infer a thin sheet-like form for an intrusion that was previously interpreted as a vertically extensive cylindrical body. The boundary of the Tallarook granite against hornfels is box-shaped in plan with sides of 5–10 km length. These sides are quite irregular on the scale of 100s of metres with many near-vertical and near-horizontal intervals that define a step-and-stair geometry. There are also intervals with moderate outward dip, and fewer inward-dipping intervals. Various parts of the exposed contact are inferred to be the roof of a granite sheet, sides, and the underside towards a floor. The near-horizontal intervals that delineate the roof of the granite are from 200 to 800 m above sea level, with the lowest elevation of the roof being where the Goulburn River cuts through the Tallarook granite. Late magmatic features such as tourmaline, pegmatite, vugs, aplite and quartz veins, are concentrated at the inferred roof. The underside of the granite on the southwest of the Tallarook plateau is 200 to 400 m above sea level. Initial estimate is that the thickness of the Tallarook granite is at least 400 m, and could be less than 1 km. The neighbouring 10 km² Kerrisdale pluton has a 13 km outer boundary that has been mapped to reveal both flat and steep intervals. This pluton is inferred to be near-horizontal and a few hundred metres thick. This approach of detailed field mapping should be applicable to other granite intrusions though of course not all. In the example of Tallarook, detailed field mapping of the granite boundary has been effective despite the topography not being extreme and the outcrop being modest.